By Jack Kim and Ju-min Park

SEOUL (Reuters) - Sounding at times like a mouthpiece for hostile North Korea, a candidate with no chance of winning South's presidential election made the biggest impact in a televised debate and may have scuppered the chances of the main opposition candidate.

Lee Jung-hee of the pro-North Korea United Progressive Party is polling less than one percent ahead of the December 19 election, but her strident performance in the debate on Tuesday evening may well have fatally damaged the mainstream left-wing candidate Moon Jae-in, whose party is allied with hers in parliament.

Lee topped searches on South Korea's main Internet portal Naver after the debate in which she labeled the administration in Seoul the "government of the South", a term usually employed by North Korea's propaganda machine.

Lee outraged supporters of conservative contender Park Geun-hye by using the Japanese name of her father who ruled the South for 18 years until his assassination in 1979, a reference to Korea's colonial past and an effort to portray Park as a collaborator.

"Takaki Masao wrote a pledge of patriotism with his own blood so that he could become an officer in the Japanese army, you will all know who that is," Lee said during the debate late on Tuesday. "His Korean name is Park Chung-hee."

Park appeared to ignore the attack during the debate. Her father's harsh rule saw political repression accompany rapid economic development and still divides South Koreans.

Park has held a small but steady lead over Moon Jae-in for most of the campaign, but her advantage has been within the margin of error for most polls.

"Moon was the victim in the debate," said Ka Sang-joon, Dankook University political science professor, who added that Moon was forced to hold back from attacking Park's policies as he risked appearing to bully her together with Lee.

Lee also said that North Korea was entitled to sink a South Korean naval vessel in 2010 and to shell an island, killing civilians for the first time in a direct military attack on South Korean soil for the first time since the 1950-53 Korean War.

Lee, who will turn 43 three days after the vote, was invited to take part in the presidential debate as her party has more than five parliamentary seats.

A graduate of the elite Seoul National University law school who came near the top in South Korea's tough college entrance exams, Lee has frequently courted controversy, defending North Korea's dynastic succession and saying the responsibility for ending the North's isolation rests with the United States.

"The United States and North Korea have long been in conflict," she once wrote. "The start to better ties would be that the United States could offer positive views on North Korea's leader."

(Editing by David Chance and Robert Birsel)